Method and system for producing a shell mould, in particular for investment casting

ABSTRACT

Disclosed are a method and a system for producing shell moulds, in particular for investment casting. The system  10  comprises a slipping device  14 , a sanding device  12  and a drying device  16 . The drying device  16  has a drying chamber  30  for drying a slipped pattern  20 . Infrared light sources  34  are arranged in the drying chamber  30 . The temperature within the drying chamber is adjusted above 25° C. using the infrared light sources  34  or using separate heating means.

FIELD OF THE INVENTION

The invention relates to the field of the production of casting moulds.The invention relates, in particular, to the production of shell mouldsby applying one or more layers to a pre-fabricated pattern.

BACKGROUND OF THE INVENTION

Ceramic shell moulds are often used in casting methods such as, forexample, investment casting. These shell moulds are produced by applyingone or more layers to a pattern of the subsequent cast part. Theindividual layers applied to the pattern contain a slip and a material,generally a granular material such as sand, applied to the slip. In thefirst layer applied to the pattern, the addition of the granularmaterial may also be dispensed with. Once one layer has been applied, itis dried before the following layer may be applied or—after the finallayer has dried—the pattern may be removed. As a result of thesuccessive application of the individual layers, a shell surrounding thepattern is gradually produced.

Once the final layer has been applied, the pattern is removed from theshell and the shell is then burnt. The pattern may be removed from theshell in various ways. If, for example, it is a pattern made of wax, thewax is removed by means of melting-off. If, on the other hand, thepattern is made of a thermoplastic material, the plastics material hasto be burnt out from the shell.

The drying of the individual layers applied to the patternconventionally takes place at ambient temperature, wherein care is takenthat the water contained in a newly applied layer is removed promptlybut not instantaneously. Drying usually takes place at approximately 21to 23° C. and at a relative air moisture content of more thanapproximately 40%. In order to shorten the drying process, it isrecommended to expose the respective layer to be dried to an air flow.The air flow assists the removal of the moisture as it evaporates.

One drawback of the conventional drying method is the comparatively longdrying period, usually of three to more than ten hours per layer. Thisis caused by the low diffusion gradient within the layer applied last.Even if the drying period is significantly extended, the residualmoisture in the applied layers may not be reduced as required.Especially in the lower zones of the layer applied last, the remainingmoisture tends more to diffuse back into the adjacent layer, which haspresumably dried, than to evaporate.

For the aforementioned reasons, a certain residual moisture is alwayscontained in the shell, even after the final drying process. Thisresidual moisture hinders and restricts the desired irreversible bondingof the colloids contained in the slip. Moreover, in the case ofreversible colloid bonds, moisture (for example, from the atmosphere)which acts after the end of the drying process can impair the compositeconstruction of the shell by detaching the reversible bonds.

If there is insufficient irreversible colloid bonding, there is therisk, during melt-off or burning-out of the pattern, that the materialof the pattern, as a result of its heat-induced expansion, will breakthe shell. The less complete the irreversible colloid bonding is, thegreater is this risk. Although this risk may be reduced in that theshell mould is subjected to a thermal shock (for example, in ahigh-pressure steam autoclave), the water vapour thereby used causes theshell mould to be penetrated again by moisture, with correspondinglynegative effects on the strength of the mould.

In order to assist the drying process, GB 2 350 810 A proposes to admixwater-insoluble organic fibres to the slip. The admixing of organicfibres has a positive effect on the drying period and also allows theresidual moisture to be reduced. These positive influences are caused bythe capillary effect of the admixed fibres, which assists the removaland the evaporation of the moisture. The fibre composite structure alsoproduces a more uniform layer construction and allows the layerthickness to be increased.

Despite the positive effects of the admixing of organic fibres, thedrying period of individual layers is often still too long. In the caseof multilayered shells, in particular, it is therefore almost impossibleto produce a shell mould ready for casting in a single day. This may beacceptable in the case of industrial applications, in which shell mouldsare produced continuously; however, in a large number of otherapplications, such as the production of prototypes, it would appeardesirable to reduce the production period for an individual shell mould.

The object of the invention is to specify a method and a system for themore rapid production of a shell mould.

SUMMARY OF THE INVENTION

The invention provides a method for producing a shell mould (inparticular for investment casting) that involves the steps of providinga pattern, forming a shell surrounding the pattern, by applying at leastone aqueous layer to the pattern and by carrying out, layer by layer, atleast one drying process, and removing the pattern from the shell, thedrying process being carried out above a temperature of 25° C. andassisted by infrared light radiation.

The layer applied to the pattern may be a layer containing anincombustible slip. The layer may also contain an incombustible granularmaterial. However, according to a preferred variant of the invention, atleast the first layer, which is applied directly to the pattern, doesnot contain any granular material. The slip may contain anincombustible, liquid binder such as, for example, an aqueous silicasol. The slip may also comprise an incombustible powder.

According to a first variant of the invention, in the case of amultilayered shell construction, each individual layer is subjected to adrying process according to the invention. According to a second variantof the invention, individual layers are either not dried (or in any casenot dried completely) or else dried at a temperature of 25° C. or lessand/or without infrared light radiation.

The process for drying an individual layer may take place at asubstantially constant temperature or at a varying temperature. Thedrying process may be carried out at a temperature above 28° C. or above30° C., and expediently in a temperature range of up to approximately45° C. A temperature range from approximately 36° C. to approximately42° C. is preferred.

If a plurality of layers are applied to the pattern, the (maximum)drying temperature may vary from layer to layer. The maximum dryingtemperature may thus substantially increase from layer to layer. As aresult of the cooling associated with the evaporation of the moisture,it is possible to select the maximum drying temperature (ambienttemperature) during the drying process such that it is above atemperature at which the pattern might lose its dimensional stability.The maximum drying temperature may therefore be at least approximately5° C. (preferably at least approximately 8° C. or 10° C.) above thetemperature at which a decrease in the stability of the pattern mightstart.

During the drying process, a relative rotation may take place betweenthe coated pattern and at least one infrared light source. This relativerotation takes place, for example, at a speed between 0.5 and 8 rpm,preferably between 1.5 and 4 rpm.

The drying process may also be assisted by means of a stream of agaseous medium such as air. The streaming rate of the gaseous medium is,for example, approximately 0.5 to approximately 8 m/s, and preferablybetween approximately 1 and approximately 5 m/s. The drying process mayalso be assisted in that the ambient moisture content is less than 35%or less than 30%. According to a preferred variant of the invention, theambient air moisture content is less than approximately 20% or less thanapproximately 10%.

The method according to the invention allows the drying period to beshortened. The process for drying a single layer may thus take less thanone hour, preferably approximately 25 to 45 min. If three or more layersare applied to the pattern, the drying period for at least some of thelayers applied after the first layer may be varied. The drying period ofthe second and/or the third layer and/or the fourth layer may thereforebe selected so as to be longer than the drying period of the otherlayers and, in particular, of the following layers.

The drying period may be adjusted as a function of a desired degree ofdryness. According to a first variant, a plurality of layers are appliedto the pattern and the individual drying process is in each case carriedout until complete drying of the layer applied last has been achieved.Complete drying may be assumed if, for example, the residual moisturecontent of a layer is less than approximately 60% and preferably betweenapproximately 55 and approximately 40%. According to a second variant,some layers, a plurality of layers or all of the layers are dried onlypartially.

The pattern used for producing the shell may be made of variousmaterials (for example, of wax or of a thermoplastic material such asABS). In the case of a wax pattern, the melting-off from the dried shellmay take place at a temperature of more than approximately 140°,preferably at approximately 150°.

The method according to the invention is suitable for a large number ofhighly diverse applications. Owing to the short drying period, themethod is thus particularly suitable, for example, for the production ofprototypes by means of investment casting (i.e. for the production ofsome or a few cast parts). However, the method is also suitable forindustrial batch processes (using, for example, a conveying deviceconfigured as a chain conveyor).

In addition to the above-mentioned method, the invention also includes asystem for producing a shell mould. The system contains a slippingdevice for applying a slip layer to a pattern and a drying device fordrying the slip layer applied to the pattern, the drying devicecomprising a drying chamber and at least one infrared light sourcearranged in the drying chamber, wherein a temperature of more than 25°C. may be adjusted in the drying chamber. A suitable adjustment orcontrol means, which ensures (for example, in a program-controlledmanner) that the desired drying temperature or the desired dryingtemperature characteristic and the subsequent drying temperature areadhered to, may be provided for adjusting the drying temperature.

The heat energy required for achieving the drying temperature may besupplied by the infrared light source. In this case, the infrared lightsource may act as a means for heating the drying gas (for example, air).For adjusting the desired drying temperature, the energy consumption ofthe infrared light source may be monitored in a suitable manner.Additionally or alternatively, it is conceivable to provide a separatecooling means. The cooling means may, for example, be configured suchthat it allows the supply of a cooling gas into the drying chamber. Itwould also be conceivable to provide an additional heating means,separately from the infrared light source.

The system may comprise a means for rotating the coated pattern withrespect to the at least one infrared light source. Such relativerotation between the coated pattern and the infrared light sourceensures more uniform surface heating and therefore improves the layerquality. A sanding device for sanding the slip layer applied to thepattern may also be provided. The sanding device is configured forapplying granular material (not necessarily sand) to the slip layer in amanner known per se.

For automating the production of shell moulds, a conveying device, whichmoves the pattern (back and forth in the case of a multilayeredconstruction) between the slipping device and the drying device, may beprovided. The conveying device may also ensure that the pattern isconveyed to or from the sanding device. The conveyance direction isexpediently selected such that the slipping device is located before thesanding device, and the sanding device before the drying device.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and details of the invention will become apparentfrom the following description of preferred embodiments and from thedrawings, in which:

FIG. 1 is a side view of a system according to the invention forproducing a shell mould;

FIG. 2 is a plan view of the system according to FIG. 1;

FIG. 3 is a side view of the drying device of the system illustrated inFIGS. 1 and 2;

FIG. 4 is a front view of the drying device according to FIG. 3;

FIG. 5 is a plan view of the drying device according to FIG. 3;

FIG. 6 shows the results of a bending test for a ceramic plate driedaccording to the invention; and

FIG. 7 shows the results of a bending test for a ceramic plate dried ina conventional manner.

DESCRIPTION OF PREFERRED EMBODIMENTS

A system 10 according to the invention for producing shell moulds willbe described below, initially with reference to FIG. 1 to 5. The methodaccording to the invention will then be explained, using variousexamples, and contrasted with a comparative example.

FIGS. 1 and 2 schematically show the system 10 according to theinvention for the automated production of shell moulds. The system 10allows the method steps of slipping, sanding and drying to be carriedout. A sanding device 12, a slipping device 14 and a drying device 16are accordingly provided. The system 10 also comprises a device 18 forconveying a pattern 20.

For the mode of operation of the system 10, it is immaterial whether thepattern 20 has not yet been coated or has already been provided with oneor more layers. For the purposes of illustration, the pattern 20 isshown in FIG. 1 in four different procedural states simultaneously,namely within the sanding device 20, within the slipping device 14,within the drying device 16 and in a conveyance state. Duringconventional operation of the system 10, the pattern 20 will be in onlyone of these four states illustrated in FIG. 1, as the system 10 isconfigured for rapid prototype production and not for industrial batchprocesses. Nevertheless, by providing a plurality of the individualdevices 12, 14 and 16 and correspondingly reconfiguring the conveyingdevice 18 (for example, as a chain conveyor), the system 10 could alsobe configured for batch processes.

In the case of the embodiment illustrated in FIG. 1, the sanding device12 is configured as a sand drum, in which sand or another granularmaterial is scattered onto the rotating pattern 20, which is providedwith a slip layer. In the embodiment, the slipping device 14 is a slipvat, which is filled with a suitable slip. The pattern 20 may beimmersed into the slip vat 14, and rotated therein, by means of theconveying device 18.

A pattern 20 received by the conveying device 18 may be supplied to theslip vat 14, the sand drum 12 or the drying device 16, as desired. Theconveying device 18 itself comprises a receiver head 22, which ismovable along an x-axis and a y-axis, for receiving the pattern 20. Thereceiver head 22 is rotatable about two axes extending perpendicularlyto each other, as indicated in FIG. 1 by the arrows 24, 26.

In conventional use of the system 10, the pattern 20 is initiallyimmersed into the slip vat 14, and the slipped pattern 20 (in particularif it is the first slip layer) then either immediately dried in thedrying device 16 or else firstly sanded in the sand drum 12, and onlythen transferred into the drying device 16.

FIG. 3 to 5 are various views of the drying device 16. As may be seenfrom these figures, the drying device 16 comprises a drying chamber 30.A large number of fans 32, which are arranged in a plurality of opposingrows, and a plurality of infrared light sources 34 are arranged in thedrying chamber 30. The fans 32 cause air circulation and produce an airflow, which assists the drying process. As may clearly be seen in FIG.4, the fans accelerate the air tangentially with respect to an imaginarycylindrical element 36. The heat energy generated by the infrared lightsources 34 causes heating of the air circulating in the drying chamber30. The infrared light sources 34 therefore act as heating means. Foruniform heating of the surface of the coated pattern 20 by the infraredlight sources 34, the pattern 20 is continuously rotated within thedrying chamber 30.

The drying device 16 also comprises an air-conditioning device 38 forcooling intake air. The air-conditioning device 38 issues warmdischarged air and supplies cooling air to an air drier 40. Thiscircumstance is illustrated by two arrows. The air drier 40, which isbased on an absorption drying principle, introduces dry supply air intothe drying chamber 30 and issues moist discharged air to theenvironment. This circumstance is also illustrated by two arrows. As maybe seen in FIG. 5, a principal cycle 46, which results substantiallyfrom the constant air circulation caused by the fans 32, forms withinthe drying chamber 30. A secondary cycle 48, which encompasses the airdrier 40, also occurs. The principal cycle 46 and the secondary cycle 48are mixed in a mixing chamber 50. This mixing ensures a reduction in themoisture content of the air in the principal cycle 46, as moist air fromthe principal cycle 46 enters the secondary cycle 48 and, from there,the air drier 40. The mixing also causes cooling of the air in theprincipal cycle 46, as cooled air, which the air drier 40 feeds into thesecondary cycle 48, is continuously supplied to the air drier 40 by theair-conditioning device 38.

The air-conditioning device 38 is activated in such a way that thedesired drying temperature is set in the drying chamber 30. Theair-conditioning device 38 therefore counteracts the heating, whichoriginates from the infrared light sources 34, of the drying air. Aseparate heating means may, if necessary, be provided in addition to theinfrared light sources 34 (for example, the air-conditioning device 38could also be configured to supply warm air to the air drier 40). Unlikein FIG. 3 to 5, the air-conditioning device 38 may also communicate(additionally or exclusively) with the mixing chamber 50. Theair-conditioning device 38 (and to a certain extent the air drier 40)thus allows a desired drying temperature to be set. For this purpose,the air-conditioning device 38 and the air drier 40 may be coupled to asuitable control or adjustment device (not shown), which influences, ina program-controlled manner, the drying parameters in the drying chamber30.

It should be noted at this point that the arrangements, illustrated inFIG. 3 to 5, of the fans 32 and the infrared light sources 34 areintended merely by way of example. It would therefore be conceivable, inparticular, to arrange the infrared sources 34, which are used not onlyfor surface heating but also for heating the air, on additional sides ofthe pattern 20. The infrared light sources 34 may, for example, bearranged in (two or more) rows, which oppose one another with respect tothe pattern 20, so that they issue the infrared radiation substantiallyperpendicularly to the pattern 20.

Various specimens of ceramic moulds were produced and tested, using waxpatterns, by means of the system described with reference to FIG. 1 to5. All of the specimens were produced using a slip containing a bindersuspension consisting of WEXCOAT from Wex Chemicals, Greenford, London,England (having an SiO₂ content of 24%), a content of 1 to 5% organicfibres having a length of 1 mm, and molochite powder (−200 mesh). Theviscosity of the slip was initially 41 s (measured using the WEX beakermethod). The patterns were immersed into the slip vat 14 forapproximately 10 s and then—apart from the first layer—sanded withmolochite grains (diameter 0.3 to 0.5 mm) in the sand drum 14. Thecoated patterns were then subjected layer by layer to a drying process,assisted by means of infrared light radiation, in the drying device 16.

The wax pattern used had a cubic shape, in which a blind hole having adiameter of 20 mm and a depth of 20 mm was formed. The temperature andmoisture surface values specified in the following tables were measuredduring the drying process inside this blind hole.

A first pattern tree was provided with a total of six layers (or—in thedried state—coats), wherein the first layer was not subjected tosanding. Each individual layer was dried completely in a separate dryingprocess. The individual drying processes were carried out at a streamingrate of approximately 1.5 m/s, with constant infrared light radiation.The maximum drying temperature gradually increased from layer to layer.A drying process was considered to have been completed once the residualmoisture content measured on the surface was less than approximately55%. During the drying process, the specimen was rotated with respect tothe infrared light sources at a rotational speed of approximately 2.5rpm. The moisture content of the air in the drying chamber was graduallyreduced. Care was taken that the air moisture content was, as far aspossible, always less than approximately 20% and the temperature alwaysabove approximately 30° C.

The total process time and the individual drying parameters and surfaceconditions for each layer in one of the initial tests (in which themoisture content in the air in the drying chamber was stillcomparatively high) may be inferred from the following table. The dryingparameters and surface conditions were measured two to five times foreach drying process. Ceramic mould I Drying conditions Surfaceconditions Time Dry time Tem- Moisture Tem- Moisture [hours: [hours:perature content perature Content min] min] Coat [° C.] [%] [° C.] [%]0:00 0:10 Without 29 19 sand 0:10 0:00 Second 29 22 0:15 0:05 layer 3038 0:25 0:15 33 25 0:30 0:20 34.5 19 0:33 0:00 Third 34.1 17 0:38 0:05layer 33.6 22 0:43 0:10 33.5 22 0:58 0:25 33.8 21 1:03 0:30 34 20 24 531:07 0:00 Fourth 34 19 1:17 0:10 layer 34.2 19 1:27 0:30 34.7 19 25 602:05 0:58 35.7 18 35 47 2:10 0:00 Fifth 36 18 2:55 0:45 layer 38.1 163:05 0:00 Sixth 38 15 3:35 0:30 layer 38.4 14

As may be seen from Table 1, the total processing time of all six layerswas 3 hours and 35 minutes. The drying time alone was approximately 3hours and 15 minutes. The first layer (without sand) was dried for 10minutes, the second layer had reached a surface residual moisturecontent of approximately less than 55% after approximately

20 minutes. The corresponding drying period for the third layer wasapproximately 30 minutes, for the fourth layer approximately 58 minutes,for the fifth layer approximately 45 minutes, and for the sixth layerapproximately 30 minutes.

The thickness structure of the first ceramic mould specimen may beinferred from the following table: Increase in State Size thickness Waxpattern 35.0 mm   0 mm Coat without sand 35.5 mm 0.25 mm Second coat37.0 mm 0.75 mm Third coat 39.0 mm  1.0 mm Fourth coat 40.5 mm 0.75 mmFifth coat 42.0 mm 0.75 mm Sixth coat 44.0 mm  1.0 mm

According to this table, in the slip/sanding composition used, there wasan average layer structure of 0.8 mm per coat.

The following table shows the drying parameters and surface conditionsfor a further ceramic mould specimen having seven coats. The streamingrate in the drying chamber was approximately 1.5 to 2.0 m/s. Ceramicmould II Drying conditions Surface conditions Time Dry time Tem-Moisture Tem- Moisture [hours: [hours: perature content perature Contentmin] min] Coat [° C.] [%] [° C.] [%] 0:00 0:08 Without 33.7 17 sand 0:080:00 Second 34.3 17 0:18 0:10 layer 34.7 17 0:28 0:00 Third 35.2 15 0:580:30 layer 36.8 14 26.4 55 1:01 0:00 Fourth 36.9 14 1:31 0:30 layer 37.212 29 64 1:36 0:35 37.4 11 27 52 1:41 0:00 Fifth 37 13 2:11 0:30 layer38.2 10 27.3 58 2:16 0:00 Sixth 38.4 10 2:46 0:30 layer 38 10 27.6 482:51 0:00 Seventh 38 10 3:21 0:00 layer 38.8 8

The following two tables show corresponding measurements taken on twoidentical ceramic mould specimens, each comprising eight layers and withdrying at a streaming rate between 2 and 4 m/s. The viscosity of theslip used in these specimens was approximately 38 s. Drying conditionsSurface conditions Time Dry time Tem- Moisture Tem- Moisture [hours:[hours: perature content perature Content min] min] Coat [° C.] [%] [°C.] [%] Ceramic mould III/1 0:00 0:08 Without 35.9 17 sand 0:10 0:00Second 36.5 17 layer 0:30 0:00 Third 38.6 14 1:20 0:50 layer 29 70 1:250:55 60 1:30 1:00 28.6 45 1:30 0:00 Fourth 37.4 16 2:00 0:30 layer 722:05 0:35 27.8 42 2:10 0:00 Fifth 39 15 2:40 0:30 layer 27.9 45/52 2:430:00 Sixth 3:13 0:30 layer 38.9 16 76 3:18 0:35 59 3:25 0:00 Seventh3:55 0:30 layer 65/70 4:00 0:35 48/52 4:05 0:00 Eighth 5:00 0:55 layerComplete Ceramic mould III/2 0:00 0:12 Without 37.8 12 sand 0:12 0:00Second 37 10 0:32 0:20 layer 39.1 15 27.5 45/59 0:37 0:25 42/58 0:420:00 Third 38.9 14 1:20 0:38 layer 39.2 14 29 75 1:32 0:50 42/61 1:370:55 39.2 9 29.8 40/52 1:42 0:00 Fourth 38.9 11 layer 2:32 0:00 Fifth39.8 10 3:02 0:30 layer 30.5 48/61 3:07 0:35 30.4 44/53 3:12 0:00 Sixth39.5 16 3:42 0:30 layer 44/50 3:42 0:00 Seventh 38.2 9 4:12 0:30 layer38 13 30 48/52 4:12 0:00 Eight 4:42 0:30 layer 29.8 47Immediately melted off

In the aforementioned ceramic mould specimens, the melting-off of thewax pattern took place immediately after the application and drying ofthe final layer. Melting-off took place in a hot cabinet preheated to150°. The wax had completely melted off after, in each case 15 to 20minutes. A visual inspection revealed that the specimens produced in thedrying chamber could be melted off without any damage or cracks.

A ceramic mould specimen produced at the same time under conventionaldrying conditions (see the following table) was completely destroyed,under the selected melting-off conditions, by cracks. The comparativespecimen was produced in the same manner as the above ceramic moulds, bymeans of repeated slipping, sanding and drying. However, drying tookplace under conventional drying conditions (no drying chamber was used)and without infrared light radiation, but with accelerated ambient air(1.5 m/s). Comparative specimen Drying conditions Surface conditionsTime Dry time Tem- Moisture Tem- Moisture [hours: [hours: peraturecontent perature Content min] min] Coat [° C.] [%] [° C.] [%] 0:00 0:30Without 22 54 sand 0:30 0:30 Second 1:30 1:00 layer 22.5 55 1.30 0:00Third 2:30 1:00 layer 25.4 43.7 23.6 67 4:00 2:30 4.15 0:00 Fourth 24.548 5:30 1:15 layer 25.0 66 6:15 2:00 25.5 65 6:45 2:30 25.0 69 7:15 3:0025.6 46 7:20 0:00 Fifth 24.0 51.5 layer One day later 21:30 0:00 Sixth21.0 46 23:00 1:30 layer 23.1 43.2 22.8 66 23:30 2:00 24.0 40.7 23.3 7024:30 3:00 23.0 58 24:30 0:00 Seventh 25:30 1:00 layer 22.4 42.4 27:303:00 25.0 41 25.0 50 27:45 0:00 Eighth 30:45 3:00 layer 26.0 62Real time in this case: 17:15; melting-off 24 hours later

As may be seen from the above table, the drying periods aresignificantly longer in the case of the comparative specimen than in thecase of the specimens produced using the method according to theinvention.

The strength of the specimens according to the invention is alsosignificantly greater than the strength of the conventional specimens.Test strips having the dimensions 50 mm 20 mm×5 mm were produced inorder to determine the bending strength of the ceramic. A silicone mouldcomprising a plurality of bowl-like indentations was used for producingthe test strips. For applying a plurality of coats, the silicone mouldwas repeatedly slipped, sanded and dried. Six to eight coatings weretypically provided in order to achieve a strip height of approximately 5mm.

The test strips according to the invention were subjected (in some casessimultaneously with pattern trees) to a drying process in the dryingchamber at a temperature of approximately 40° C., an air moisturecontent of approximately 5 to 10% and a drying period of approximately30 min. During the drying process, the strips were irradiated withinfrared light. The conventional test strips, on the other hand, weredried at ambient temperature and an air moisture content ofapproximately 50%. Each layer was dried until the surface moisturecontent was less than 60% (this typically took several hours to oneday). All of the test strips were then subjected to a bending test. The7/18 strength-testing device from Feinmechanik Ralf Kögel was used forthis purpose.

FIG. 6 shows the results of the test for the test strips dried in themanner according to the invention, and FIG. 7 the results of the testfor conventional test strips (two green parts and two burnt specimenswere checked in each case; the test strips were burnt for one hour at950° C.). A comparison of the respective characteristic clearlyindicates that the load capacity of the specimens according to theinvention exceeds the load capacity of the specimens dried in theconventional manner, at least in the burnt state, by almost 50%. Thegreen parts dried according to the invention also exhibit asignificantly higher load capacity than the green parts dried in theconventional manner.

The basis for the advantages according to the invention is believed tobe that the ion exchange at the surface of the binder colloids isintensified at relatively high drying temperatures, and this allowsstrong irreversible bonding of these colloids to one another. Theintensive, surface-based drying caused by the infrared light radiationalso results in a higher diffusion gradient within the applied sliplayer, and thus in accelerated drying. The effect of the latent heatallows the drying temperature to be increased, beyond the temperature atwhich the pattern used would lose its stability. This also allowsaccelerated drying.

Each coat layer preferably experiences complete drying, in order tobring about irreversible colloid bonding. The desired final strength ofthe overall shell is therefore achieved immediately after the end of thedrying of the final layer. In other words, it is no longer essential tocontinue waiting, once the layer applied last has dried, before themelting-off/burning-out of the model and the burning of the ceramicmould may commence. Nevertheless, this recognition does not rule outcarrying out a concluding, longer final drying process in specificcases.

For achieving particularly short drying times, it was expedient toreduce the air moisture content in the drying chamber. In a series ofconsecutive tests, the air moisture content was reduced to less than10%, often to 2% to 8%.

The tests revealed that the first sanded layer (i.e. generally thesecond layer applied to the pattern) dries relatively quickly(approximately 20 min), whereas the first or the second following layerrequires a longer-than-average time (up to 60 min) in order to drycompletely. The drying times for the subsequent layers were typically 30to 35 minutes. At the start of the drying process, the residual moisturecontent in the immersed layer often rises briefly to above 80%, thenremains for a long time at 65 to approximately 70%, in order then,approximately 2 to 10 minutes (typically approximately 5 min) before thedeterminable end of the drying period, to drop almost spontaneously toless than 50%.

When using the method according to the invention for the production ofprototypes, it is advisable to have only one slip of uniform viscosity(uniform slip) for all of the coat layers and only one grain size of thescattering material. The uniform slip results in improved wetting, whileat the same time reducing the runout time to 38 s (measured using theWEX beaker). An adequate surface quality of the cast parts may beachieved by inserting an immersion layer, without sanding, at the startof the shell construction process, while keeping the initial dryingprocess short (typically less than 15 min).

The invention has been described, by way of example, with reference tovarious embodiments. A person skilled in the art may make modificationsand additions on the basis of his specialised knowledge. The location,the positioning and the number of the infrared light sources and thearrangement and the number of the fans may therefore, in particular, bealtered.

1. A method for producing a shell mould, comprising providing a pattern,forming a shell surrounding the pattern by applying at least one aqueouslayer to the pattern and by carrying out, layer by layer, at least onedrying process, wherein the drying process is carried out above atemperature of 25° C. and is assisted by infrared light radiation, andremoving the pattern from the shell,
 2. The method according to claim 1,wherein the drying process is carried out in a temperature range fromapproximately 30° C. to approximately 45° C.
 3. The method according toeither claim 1, wherein a plurality of layers are applied to the patternand wherein the maximum drying temperature is substantially increasedfrom layer to layer.
 4. The method according to claim 1, wherein themaximum drying temperature during the drying process is selected so asto be at least approximately 5° C. above a temperature at which thestability of the pattern starts to decrease.
 5. The method according toclaim 1, wherein a relative rotation between the coated pattern and atleast one infrared light source takes place during the drying process.6. The method according to claim 5, wherein the relative rotation takesplace at a speed between 0.5 and 8 rpm.
 7. The method according to claim1, wherein the drying process is assisted by a streaming of a gaseousmedium.
 8. The method according to claim 7, wherein the streaming rateof the gaseous medium is between approximately 0.5 and approximately 8m/s.
 9. The method according to claim 1, wherein the drying process iscarried out at an ambient air moisture content of less than
 35. 10. Themethod according to claim 1, wherein the drying process is carried outfor a period of less than one hour.
 11. The method according to claim 1,wherein three or more layers are applied to the pattern and the dryingperiod for at least some of the layers applied after the first layer isvaried.
 12. The method according to claim 11, wherein the drying periodof at least one of the second, the third, and the fourth layer isselected so as to be longer than the drying period of the other layers.13. The method according to claim 1, wherein a plurality of layers areapplied to the pattern and the individual drying process is in each casecarried out until complete drying of the layer applied last is achieved.14. The method according to claim 13, wherein complete drying is assumedif the residual moisture content of a layer is less than approximately60%.
 15. The method according to claim 1, wherein the pattern is made ofwax, which, at a temperature of more than approximately 140° C., isremoved from the dried shell by means of melting-off.
 16. The methodaccording to claim 1, wherein the drying process is carried out in adrying chamber.
 17. The method according to claim 1, wherein heat energyrequired for achieving the drying temperature is supplied by at leastone infrared light source.
 18. The method according to claim 1, whereinthe method is used for the production of prototypes.
 19. A method forproducing a shell mould, comprising: providing a pattern; forming ashell surrounding the pattern by applying two or more aqueous layers tothe pattern and by carrying out, layer by layer, a drying process abovea temperature 25° C. and below a temperature of 45° C. and assisted byinfrared light radiation; removing the pattern from the shell; whereinthe drying process is performed separately from the removal of thepattern from the shell.
 20. A system for producing a shell mould,comprising a slipping device for applying a slip layer to a pattern; anda drying device for drying the slip layer applied to the pattern, thedrying device including a drying chamber and at least one infrared lightsource arranged in the drying chamber, wherein a temperature of morethan 25° C. is adjustable in the drying chamber.
 21. The systemaccording to claim 20, wherein the infrared light source acts as a meansfor heating a drying gas.
 22. The system according to claim 20, whereina cooling device is provided for supplying a cooled drying gas into thedrying chamber.
 23. The system according to claim 20, wherein inaddition to the infrared light source, a device for heating the dryingchamber is provided.
 24. The system according to claim 20, wherein adevice is provided for rotating the coated pattern with respect to theinfrared light source.
 25. The system according to claim 20, wherein asanding device is provided for sanding the slip layer applied to thepattern.
 26. The system according to claim 20, further comprising aconveying device for conveying the pattern at least between the slippingdevice and the drying device.